Symmetry in Mesoscale Circulations Explains Weak Impact of Trade Cumulus Self‐Organization on the Radiation Budget in Large‐Eddy Simulations
M. Janssens (Wageningen University & Research, TU Delft - Atmospheric Remote Sensing)
Fredrik Jansson (TU Delft - Atmospheric Remote Sensing)
Pouriya Alinaghi (TU Delft - Atmospheric Remote Sensing)
F. Glassmeier (TU Delft - Atmospheric Remote Sensing)
A.P. Siebesma (Royal Netherlands Meteorological Institute (KNMI), TU Delft - Geoscience and Remote Sensing)
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Abstract
We investigate if mesoscale self-organisation of trade cumuli in 150 km-domain large-eddy simulations modifies the top-of-atmosphere radiation budget relative to 10 km-domain simulations, across 77 characteristic, idealized environments. In large domains, self-generated mesoscale circulations produce fewer, larger and deeper clouds, raising the cloud albedo. Yet they also precipitate more than small-domain cumuli, drying and warming the cloud layer, and reducing cloud cover. Consequently, large domains cool slightly less through the shortwave cloud-radiative effect, and slightly more through clear-sky outgoing longwave radiation, for a net cooling (−0.5 W (Formula presented.)). This cooling is generally smaller than the large-domain radiation's sensitivity to large-scale meteorological variability, which is similar in small-domain simulations and observations. Hence, mesoscale self-organisation would not alter weak trade-cumulus feedback estimates previously derived from small-domain simulations. We explain this with a symmetry hypothesis: ascending and descending branches of mesoscale circulations symmetrically increase and reduce cloudiness, weakly modifying the mean radiation budget.
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